The present invention relates generally to a surface acoustic wave (SAW) device, and more particularly to a SAW device for use in communication devices and so on, which defines a cross region of electrode fingers forming interdigital transducers (IDT), and a direction of surface acoustic wave propagation (group velocity).
SAW devices are widely used in such applications as resonators, reflectors, filters and so on as lightweight, small-size circuit elements for communication devices. The SAW devices may be classified into SAW devices which utilize Rayleigh waves, SAW devices which utilize Love waves, and so on. The SAW devices utilizing Love waves are known to have a larger electro-mechanical coupling coefficient (k2) than the SAW devices utilizing Rayleigh waves.
A known Love-wave type SAW resonator (Love-wave type SAW device) has the structure illustrated in FIGS. 1 and 3. FIG. 1 is a top plan view illustrating the basic structure of electrodes which form part of the Love-wave type SAW resonator. An interdigital transducer composed of bus bars 2-1, 2-2, electrode fingers 3, and input/output terminals 4, 5 is formed on a surface of a piezoelectric substrate 1. In the SAW resonator of FIG. 1, an electrode cross region 8 is arranged in a grating region 9 such that the width W thereof is constant in a surface wave ropagation direction. The electrode fingers connected to the bus bar 2-1 and the electrode fingers connected to the bus bar 2-2 are arranged alternatively in the electrode cross region 8. Non-harmonic higher-order longitudinal and transversal mode oscillations exist in the surface wave propagation direction, i.e., the group velocity direction (the direction indicated by the arrow in FIG. 1) and in the direction perpendicular to the surface wave propagation direction, respectively. The impedance characteristic of the resonator as illustrated in FIG. 1 disadvantageously exhibits a spurious response (indicated by a wavy line 30 in FIG. 2) resulting from the existence of such oscillations (standing waves), as can be seen in a Smith chart of FIG. 2.
A SAW resonator employing an apodization technique, as illustrated in FIG. 3, has been proposed in order to suppress the spurious response due to the nonharmonic higher-order modes (for example, see JP-A-6-85602). Referring specifically to FIG. 3, the apodization technique distributes a cross region 8a of electrode fingers 3 in conformity to a function which defines a maximum cross width at the center of the cross region 8a, and reduces the cross width along the surface wave propagation direction (indicated by the arrow in FIG. 3) to substantially zero at paired electrode fingers located at both side ends of the cross region 8a. Thus, the spurious response can be suppressed by such a apodization-based arrangement (see, for example, JP-A-6-85602, or xe2x80x9cSmall-Size Love-Type SAW Resonator with Very Low Capacitance Ratioxe2x80x9d by Hiroshi Shimizu and Yuji Suzuki, Transactions of the Institute of Electronics, Information and Communication Engineers A, Vol. j.75-A, No. 3, pp 458-466, March 1992).
A grating region 9a located outside the cross region 8a of the electrode fingers between bus bars 2-1, 2-2 also functions as a SAW waveguide. Conventionally, the influence of the SAW waveguide has hardly been taken into account. For this reason, the piezoelectric substrate and the electrode finger (grating) region 9 of electrodes have been in the shape of a rectangle, i.e., the same shape as the optical waveguide and electromagnetic wave waveguide.
The conventional SAW resonator which has been improved by the weighted cross region in accordance with the apodization technique, though it has achieved a large effect in suppressing the spurious response due to the non-harmonic higher-order modes, still fails to completely eliminate the spurious response due to the non-harmonic higher-order modes. If such a SAW resonator is used as an oscillating element for a highly accurate voltage controlled oscillator (VCO), the resulting voltage controlled oscillator suffers from skipping of the oscillating frequency caused by the spurious response due to the aforementioned non-harmonic higher-order longitudinal and transversal modes.
It is therefore an object of the present invention to realize a SAW device which is capable of further suppressing the spurious response due to the non-harmonic higher-order modes.
It is another object of the present invention to realize a SAW device which is capable of allowing for the ease of manufacturing and a reduction in size as well as achieving the above object.
To achieve the above objects, in one aspect of the present invention, a surface acoustic wave device having an interdigital transducer (IDT) formed on a piezoelectric substrate is structured such that in significant portions of boundaries between bus bars and a grating region, which is composed of electrode fingers of the IDT, the extending directions of the boundaries are oriented non-parallel with the propagation direction of acoustic surface waves.
In one aspect of the present invention, a surface acoustic wave device comprises a piezoelectric substrate, and an interdigital transducer formed on a planar surface of the piezoelectric substrate, and having first and second bus bars, a first plurality of electrode fingers connected to the first bus bar, and a second plurality of electrode fingers connected to the second bus bar, wherein the first and second plurality of electrode fingers of the interdigital transducer have an electrode cross region in which the first and second plurality of electrode fingers are arranged alternatively, and each of boundaries between the first and second bus bars and a grating region of the first and second plurality of electrode fingers is arranged such that the boundary is not substantially parallel, with a group velocity direction (transmission direction) of surface acoustic waves excited by the interdigital transducer.
In another aspect of the present invention, a surface acoustic wave device comprises a piezoelectric substrate, and an interdigital transducer formed on a planar surface of the piezoelectric substrate, and having first and second bus bars, a first plurality of electrode fingers connected to the first bus bar, and a second plurality of electrode fingers connected to the second bus bar, wherein the first and second plurality of electrode fingers of the interdigital transducer have an electrode cross region in which the first and second plurality of electrode fingers are arranged alternatively, and the distance between the first and second bus bars along the first and second plurality of electrode fingers varies along a group velocity direction of the surface acoustic waves excited by the interdigital transducer.
In one example of the present invention, each of the boundaries is arranged such that the extending direction of the boundary is at an angle in a range of 45xc2x127 degrees, in a significant portion thereof, with respect to the group velocity direction of the surface modes because of the influence of distorted standing waves generated between opposing bus bars, which had not been taken into account in the prior art. Then, the inventors fabricated a variety of prototype SAW resonators, each of which had bus bars determined in shape and arrangement such that standing waves differed in frequency and phase with respect to the propagation direction of surface acoustic waves to prevent the standing waves from being generated between the bus bars, and verified the effects of these SAW resonators.
In the aforementioned conventional SAW resonator illustrated in FIG. 3, the grating region 9 has two regions. One of the two regions is the electrode cross region 8a in which the electrode fingers connected to the bus bar 2-1 and the electrode fingers connected to the bus bar 2-2 are arranged alternatively. The other of the two regions is the region 9a in which two kinds of these electrode fingers connected to the bus bars 2-1 and 2-2 are not arranged alternatively. The grating 9 is in the shape of a rectangle, where the long sides of the rectangle extend in parallel with the direction of surface acoustic wave propagation. Two boundaries between the grating region 9 and the bus bars 2-1, 2-2 function as reflecting surfaces for certain components of surface acoustic waves (components of the surface acoustic waves having a wave vector in the direction perpendicular to the propagation direction of the surface acoustic waves (the direction indicated by the arrow in FIG. 3), i.e., non-acoustic waves excited by the interdigital transducer.
In one example of the present invention, each of the boundaries is arranged such that the extending direction of the boundary is not parallel, in one half or more of the overall length thereof, with the group velocity direction of surface acoustic waves excited by the interdigital transducer.
According to one example of the present invention, the surface acoustic wave device has a cross region of the electrode fingers weighted with respect to the propagation (group velocity) direction of surface acoustic waves, wherein the piezoelectric substrate is in the shape of a rhomboid, and the bus bars are arranged along the outer periphery of the piezoelectric substrate in the shape of a rhomboid. Further, when the cross region of the weighted electrode fingers is in the shape of a rhomboid, the distance between each of envelopes of the cross region of the electrode fingers and corresponding one of the bus bars is made constant.
While the angle between each bus bar and the propagation direction of surface acoustic waves is most preferably chosen to be approximately 45 degrees, the angle may be in a range of approximately 18 to 72 degrees to effectively suppress the spurious response due to the non-harmonic higher-order modes.
The inventors of the present invention hypothesized that a weighted SAW device still suffered from the spurious due to the non-harmonic higher-order harmonic higher-order transversal mode components). Also, between the two boundaries, surface waves S1, S2, . . . of the non-harmonic higher-order transversal mode are generated between the two boundaries, as indicated by dotted lines. Since these surface waves are consistent in frequency and phase so that they will resonate to result in standing waves, the inventors found that the standing waves thus generated would be the cause of the susceptibility of the conventional SAW resonator to the spurious response due to the non-harmonic higher-order transversal mode.
Also, electrode fingers 3a, 3b at both side ends of the rectangle extend perpendicularly to the propagation direction of the surface acoustic waves, and function as reflecting surfaces for certain components of surface acoustic waves (components of the surface acoustic waves having a wave vector in the direction parallel with the direction of the surface acoustic wave propagation, i.e., non-harmonic higher-order longitudinal mode components). Thus, surface waves S3, S4, . . . of the non-harmonic higher-order longitudinal mode are generated between the two electrode fingers 3a, 3b at the two side ends, and become standing waves which consequently cause the resonance. It is thought that the standing waves thus generated are the cause of the susceptibility of the conventional SAW resonator to the spurious response due to the non-harmonic higher-order longitudinal mode.
The SAW device according to the present invention, on the other hand, is constructed such that the opposing bus bars extend in an inclined direction with respect to the direction of surface acoustic wave propagation, so that the wave motions, potentially causing the standing waves, differ in frequency or in phase, and are accordingly less prone to leading to the generation of standing waves. Stated another way, the SAW device according to the present invention is hardly susceptible to the resonance due to the non-harmonic higher-order modes, thereby making it possible to significantly reduce the spurious response.